Nanoencapsulation methods for forming multilayer thin film structures and multilayer thin films formed therefrom

ABSTRACT

A multilayer thin film structure having a reflective core particle, a dielectric layer directly encapsulating the reflective core particle, an absorber layer directly encapsulating the dielectric layer; an outer layer encapsulating the absorber layer. The multilayer thin film structure has a hue shift of less than 30° in the Lab color space when viewed at angles from 0° to 45°.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.16/264,170 filed Jan. 31, 2019, which claims the benefit of priorityunder 35 U.S.C. § 119 of U.S. Provisional Application Ser. No.62/737,567, filed on Sep. 27, 2018, all of which is incorporated hereinby reference.

FIELD

The present application is related to methods for forming multilayerthin film structures and thin film structures formed therefrom, and inparticular to nanoencapsulation methods for forming multilayer thin filmstructures and multilayer thin film structures formed therefrom.

BACKGROUND

Previously disclosed structural color multilayer thin film structurescontain layers of metal oxides having a high refractive index and thinlayers of metals as absorbers. Traditionally, these layers of thin filmsare deposited on a substrate by high vacuum deposition processes, suchas physical vapor deposition (PVD) or sputtering. Once the layers havebeen deposited on the substrate, the substrate is removed, such as byusing solvents that dissolve the substrate or by physically removing thedeposited layers from the substrate. Subsequently, the multilayer thinfilm structure may be broken into discrete particles by mechanicallypulverizing the multilayer thin film structure or by ultrasonictreatment.

The above-described process for forming multilayer thin film structuresis both time consuming and costly. For example, maintaining the highvacuum required for the lengthy deposition process is difficult and canbe expensive. In addition, the multilayer thin film structures generallymust be deposited on the substrate layer-by-layer. Accordingly, for aseven layered thin film structure, the seven layers are deposited on thesubstrate in seven distinct deposition steps. This requires asignificant amount of time, and depositing precise layers of differentmaterials can be difficult and costly.

Accordingly, more efficient and cost-effective methods for formingmultilayer thin film structures that provide ominidirectional structuralcolor are desired.

SUMMARY

According to embodiments, a method for forming a multilayer thin filmstructure comprises: directly depositing an absorber layer toencapsulate a dielectric layer, wherein the dielectric layerencapsulates a reflective core particle; and depositing an outer layerto encapsulate the absorber layer, wherein the multilayer thin filmstructure has a hue shift of less than 30° in the Lab color space whenviewed at angles from 0° to 45°.

According to embodiments, a multilayer thin film structure comprising: areflective core particle; a dielectric layer directly encapsulating thereflective core layer; an absorber layer encapsulating directlyencapsulating the dielectric layer; an outer layer encapsulating theabsorber layer, wherein the multilayer thin film structure has a hueshift of less than 30° in the Lab color space when viewed at angles from0° to 45°.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from that description or recognized by practicing theembodiments described herein, including the detailed description whichfollows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description describe various embodiments and areintended to provide an overview or framework for understanding thenature and character of the claimed subject matter. The accompanyingdrawings are included to provide a further understanding of the variousembodiments, and are incorporated into and constitute a part of thisspecification. The drawings illustrate the various embodiments describedherein, and together with the description serve to explain theprinciples and operations of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A schematically depicts a multilayer thin film structure;

FIG. 1B schematically depicts a multilayer thin film structure accordingto embodiments disclosed and described herein;

FIG. 2 depicts a multilayer thin film with a dielectric layer extendingover a substrate layer and exposed to electromagnetic radiation at anangle θ relative to a normal direction to the outer surface of thedielectric layer;

FIG. 3A is a graph showing electric field intensity versus distance innm for various wavelengths of electromagnetic radiation;

FIG. 3B is a graph showing absorbance versus wavelength for structurescomprising a Cr absorber and structures not comprising a Cr absorber;

FIG. 3C is a graph showing reflectance versus wavelength for structurescomprising a Cr absorber and structures not comprising a Cr absorber;

FIG. 4 is a schematic flow chart of methods for forming multilayer thinfilm structures according to embodiments disclosed and described herein;

FIG. 5 is a schematic of a multilayer thin film structure comprising aprotective layer according to embodiments disclosed and describedherein;

FIG. 6A is a photograph showing the change in color of multilayer thinfilm structures as a function of tungsten deposition cycles;

FIG. 6B is a magnified view of a tungsten layer deposited on reflectivecore particle coated with iron oxide;

FIG. 6C is a graph showing the thickness of a tungsten layer versusatomic layer deposition cycle;

FIG. 6D is a graph showing n values and k values versus wavelength ofelectromagnetic radiation;

FIG. 7A is a series of magnified photographs of crystalline titaniumdioxide layers deposited as layers of multilayer thin film structures;

FIG. 7B is a spectra of anatase titanium dioxide deposited as layers ofmultilayer thin film structures;

FIG. 7C is a graph showing n values and k values versus wavelength ofelectromagnetic radiation;

FIG. 7D is a graph showing thickness of titanium dioxide versus theatomic layer deposition cycle;

FIG. 8A is a schematic showing processes for forming multilayer thinfilm structures according to embodiments disclosed and described herein;

FIG. 8B is a matrix of tungsten and titanium dioxide thickness as afunction of atomic layer deposition cycles;

FIG. 8C is a graph showing reflectance versus wavelength for structures;

FIG. 8D is a magnified photograph of a multilayer thin film structureaccording to embodiments disclosed and described herein;

FIG. 9A is a schematic showing processes for forming multilayer thinfilm structures according to embodiments disclosed and described herein;

FIG. 9B is a series of magnified photographs of a multilayer thin filmstructure;

FIG. 9C is a series of magnified photographs of a multilayer thin filmstructure according to embodiments disclosed and described herein;

FIG. 10A is a photograph of multilayer thin film structures according toembodiments disclosed and described herein;

FIG. 10B is a graph showing intensity versus binding energy; and

FIG. 10C is a magnified photograph of a multilayer thin film structureaccording to embodiments disclosed and described herein.

DETAILED DESCRIPTION

In view of the high cost and lower efficiency involved in producingmultilayer thin film structures described above, embodiments ofnanoencapsulation methods for forming multilayer thin film structuresdisclosed and described herein are directed to methods that apply layersdirectly to the reflective core particle by nanoencapsulation of thereflective core—and any layers previously deposited thereon. Suchmethods significantly reduce the steps required for producing amultilayer thin film structure by reducing the number of depositionsteps. For instance, with reference to FIG. 1A, a seven layer thin filmstructure 100 comprising a first outer layer 140, a first absorber layer130, a first dielectric layer 120, a reflective core layer 110, a seconddielectric layer 120, a second absorber layer 130, and a second outerlayer 140 would require seven distinct deposition steps—one for each ofthe layers—to produce the seven layer thin film structure. In addition,the first layer to be deposited (one of the first or second outer layers140) would be deposited on a sacrificial substrate. However, as can beseen in FIG. 1A, the seven layer thin film structure 100 comprisessymmetrical layers on either side of the reflective core layer 110, suchthat the first and second dielectric layers 620 are made from the samematerial, the first and second metal layers 130 are made from the samematerial, and the first and second outer layers 140 are made from thesame material. In embodiments disclosed and described herein, and withreference to FIG. 1B, by using nanoencapsulation methods for depositingthe layers of the multilayer thin film structure 100, this process canbe reduced to three steps: (1) depositing a dielectric layer 120 thatencapsulates a reflective core particle 110 by nanoencapsulation; (2)depositing an absorber layer 130 that encapsulates the dielectric layer120 by nanoencapsulation; and (3) depositing an outer layer 140 thatencapsulates the metal layer 130 by nanoencapsulation.

Using this nanoencapsulation method for forming multilayer thin filmstructures reduces the number of deposition steps and time required toform a multilayer thin film structure can be significantly reduced. Inaddition, because the layers of the multilayer thin film are depositeddirectly onto a reflective core particle 110—as opposed to forming eachlayer of the multilayer thin film structure on a sacrificialsubstrate—no additional steps are required to remove the a sacrificialsubstrate or to mechanically or ultrasonically pulverize the formedmultilayer thin film structure into pigment particles that can be used,for example, in paints, coatings, polymers, and the like. Accordingly,nanoencapsulation methods for forming multilayer thin film structuresaccording to embodiments disclosed and described herein reduce the timeand cost involved in forming multilayer thin film layers.

There are currently a number of different approaches to deposit layersof a multilayer thin film structure over a reflective core particle,which may be a discrete metal particle or flake. These differentapproaches include, for example, wet chemical processes, chemical vapordeposition (CVD), PVD, electroless plating processes, and atomic layerdeposition (ALD) processes. Each of these deposition methods hasstrengths and weaknesses. For instance, some of the methods are costeffective, but it is difficult to deposit ultrathin layers (i.e., layersunder 50 nm in thickness). Other deposition methods are better atdepositing thin layers, but are costly, and it may still be difficult todeposit ultrathin layers of certain materials, such as metals.Accordingly, these deposition processes may be used individually or indiffering combinations to deposit one or more of the layers of themultilayer thin film structure 100.

It will be understood that the terms “electromagnetic wave,”“electromagnetic radiation,” and “light,” as used herein, mayinterchangeably refer to various wavelengths of light incidence on amultilayer thin film structure and that such light may have wavelengthsin the ultraviolet (UV), infrared (IR), and visible portions of theelectromagnetic spectrum.

Referring again now to FIG. 1B, a multilayer thin film 100 according toembodiments disclosed and described herein comprises: a reflective coreparticle 110; a dielectric layer 120 that encapsulates the reflectivecore particle 110; an absorber layer 130 that encapsulates thedielectric layer 130, and an outer layer 640 that encapsulates theabsorber layer 130.

In embodiments, the location of absorbing layers may be chosen toincrease the absorption of light wavelengths within a certain range, butreflect light in other wavelengths. For example, the location of anabsorbing layer may be selected to have increased absorption, of lightwaves less than or equal to 550 nm, but reflect light waves ofapproximately 650 nm, such as visible light outside of the hue between10° and 30°. Accordingly, the absorbing layer is placed at a thicknesswhere the electric field (|E|²) is less at the 550 nm wavelength than atthe 650 nm wavelength. Mathematically, this can be expressed as:

|E ₅₅₀|² <<|E ₆₅₀|²  (1)

and preferably:

|E ₆₅₀|²≈0  (2)

FIG. 2 and the following discussion provide a method for calculating thethickness of a zero or near-zero electric field point at a givenwavelength of light, according to embodiments. For the purposes of thepresent specification, the term “near-zero” is defined |E|²≤10. FIG. 2illustrates a multilayer thin film with a dielectric layer 4 having atotal thickness “D”, an incremental thickness “d” and an index ofrefraction “n” on a substrate layer 2 having an index of refraction“n_(s)”. The substrate layer 2 can be a core layer or a reflective corelayer of a multilayer thin film. Incident light strikes the outersurface 5 of the dielectric layer 4 at angle θ relative to line 6, whichis perpendicular to the outer surface 5, and reflects from the outersurface 5 at the same angle θ. Incident light is transmitted through theouter surface 5 and into the dielectric layer 4 at an angle θ_(F)relative to the line 6 and strikes the surface 3 of substrate layer 2 atan angle θ_(s). For a single dielectric layer, θ_(s)=θ_(F) and theenergy/electric field (E) can be expressed as E(z) when z=d. FromMaxwell's equations, the electric field can be expressed for spolarization as:

E ^(ω)(d)={u(z),0,0}exp(ikαy)|_(z=d)  (3)

and for p polarization as:

$\begin{matrix}{{E^{\omega}(d)} = \left. {\left\{ {0,{u(z)},{{- \frac{\alpha}{\overset{\sim}{ɛ}(z)}}{v(z)}}} \right\}{\exp\left( {{ik}\;\alpha\; y} \right)}} \right|_{z = d}} & (4)\end{matrix}$

where

${k = \frac{2\pi}{\lambda}},$

λ is a desired wavelength to be reflected, α=n_(s) sin θ_(s) where “s”corresponds to the substrate in FIG. 5, and {tilde over (ε)}(z) is thepermittivity of the layer as a function of z. As such:

|E(d)|² =|u(z)|² exp(2ikαY)|_(z=d)  (5)

for s polarization, and

$\begin{matrix}{\left| {E(d)} \right|^{2} = \left. {\left\lbrack \left. {{{u(z)}}^{2} +} \middle| {\frac{\alpha}{\sqrt{n}}{v(z)}} \right|^{2} \right\rbrack{\exp\left( {2ik\alpha y} \right)}} \right|_{z = d}} & (6)\end{matrix}$

for p polarization.

It should be appreciated that variation of the electric field along theZ direction of the dielectric layer 4 can be estimated by calculation ofthe unknown parameters u(z) and v(z), where it can be shown that:

$\begin{matrix}{\begin{pmatrix}u \\v\end{pmatrix}_{z = d} = {\begin{pmatrix}{\cos\;\varphi} & {\left( {i/q} \right)\sin\;\varphi} \\{{iq}\;\sin\;\varphi} & {\cos\;\varphi}\end{pmatrix}\begin{pmatrix}u \\v\end{pmatrix}_{{z = 0},{substrate}}}} & (7)\end{matrix}$

where ‘i’ is the square root of −1. Using the boundary conditionsu|_(z=0)=1, v|_(z=0)=q_(s), and the following relations:

q _(s) =n _(s) cos θ_(s) for s-polarization  (8)

q _(s) =n _(s)/cos θ_(s) for p-polarization  (9)

q=n cos θ_(F) for s-polarization  (10)

q=n/cos θ_(F) for p-polarization  (11)

φ=k·n·d cos(θ_(F))  (12)

u(z) and v(z) can be expressed as:

$\begin{matrix}{{\left. {u(z)} \right|_{z = d} = {\left. u \middle| {}_{z = 0}{{\cos\;\varphi} + v} \middle| {}_{z = 0}\left( {\frac{i}{q}\sin\;\varphi} \right) \right. = {{\cos\;\varphi} + {\frac{iq_{s}}{q}\sin\;\varphi}}}}{and}} & (13) \\{\left. {v(z)} \right|_{z = d} = {\left. {iqu} \middle| {}_{z = 0}{{\sin\;\varphi} + v} \middle| {}_{z = 0}{\cos\;\varphi} \right. = {{{iq}\;\sin\;\varphi} + {q_{s}\cos\;\varphi}}}} & (14)\end{matrix}$

Therefore:

$\begin{matrix}{\left| {E(d)} \right|^{2} = {{\left\lbrack {{\cos^{2}\varphi} + {\frac{q_{s}^{2}}{q^{2}}\sin^{2}\varphi}} \right\rbrack e^{2ik\alpha y}} = {\left\lbrack {{\cos^{2}\varphi} + {\frac{n_{s}^{2}}{n^{2}}\sin^{2}\varphi}} \right\rbrack e^{2ik\alpha y}}}} & (15)\end{matrix}$

for s polarization with φ=k·n·d cos(θ_(F)), and:

$\begin{matrix}{\left| {E(d)} \right|^{2} = {\left\lbrack {{\cos^{2}\varphi} + {\frac{n_{s}^{2}}{n^{2}}\sin^{2}\varphi} + {\frac{\alpha^{2}}{n}\left( {{q_{s}^{2}\cos^{2}\varphi} + {q^{2}\sin^{2}\varphi}} \right)}} \right\rbrack = {\quad\left\lbrack {{\left( {1 + \frac{\alpha^{2}q_{s}^{2}}{n}} \right)\cos^{2}\varphi} + {\left( {\frac{n_{s}^{2}}{n^{2}} + \frac{\alpha^{2}q^{2}}{n}} \right)\sin^{2}\varphi}} \right\rbrack}}} & (16)\end{matrix}$

for p polarization where:

$\begin{matrix}{\alpha = {{n_{s}\sin\;\theta_{s}} = {n\;\sin\;\theta_{F}}}} & (17) \\{q_{s} = \frac{n_{s}}{{\cos\;\theta_{s}}{and}}} & (18) \\{q_{s} = \frac{n}{\cos\;\theta_{F}}} & (19)\end{matrix}$

Thus for a simple situation where θ_(F)=0 or normal incidence, φ=k·n·d,and α=0:

$\begin{matrix}{{{{E(d)}}^{2}\mspace{14mu}{for}\mspace{14mu} s\text{-}{polarization}} = {{{{E(d)}}^{2}\mspace{14mu}{for}\mspace{14mu} p\text{-}{polarization}} = {\left\lbrack {{\cos^{2}\varphi} + {\frac{n_{s}^{2}}{n^{2}}\sin^{2\;}\varphi}} \right\rbrack = {\quad\left\lbrack {{\cos^{2}\left( {k \cdot n \cdot d} \right)} + {\frac{n_{s}^{2}}{n^{2}}{\sin^{2}\left( {k \cdot n \cdot d} \right)}}} \right\rbrack}}}} & \begin{matrix}(20) \\(21)\end{matrix}\end{matrix}$

which allows for the thickness “d” to be solved for (i.e., the positionor location within the dielectric layer where the electric field iszero). It should be appreciated that the thickness “d” can also be thethickness of the outer layer 640 extending over the third layer 630 thatprovides a zero or near zero electric field at the interface between theouter layer and the third layer 630. It should also be appreciated thatthe above equations can be tailored to absorb and reflect light in otherwavelengths.

With reference again to FIG. 1A, an exemplary multilayer thin filmstructure 110 may comprise an aluminum reflective core layer 110, a zincsulfide (ZnS) dielectric layer 120 across the reflective core layer 110,a chromium (Cr) absorber layer 130 across the dielectric layer 120, anda ZnS outer layer 140 across the absorber layer 130. Calculated electricfield intensity along the dielectric layer thickness for this structureis shown in FIG. 3A for various wavelengths. FIG. 3A shows the presenceof near zero energy or |E|_(d)=d₀=0 at some locations. For a specificwavelength when a thin absorber layer is placed at this point (d₀) thethin absorber layer does not absorb any electromagnetic radiation atthat wavelength, but the thin absorber layer located at this point (d₀)does absorb electromagnetic radiation at other wavelengths that d₀ nothave near zero energy at this point (d₀). As an example, and withreference to FIG. 3A, at a wavelength of 434 nm, which corresponds to ablue color), a location of a thin metal absorber layer may be selectedso that |E|_(d)=d₀=0 and the electromagnetic radiation at 434 nm (suchas blue light) is not absorbed by the thin metal absorber layer and istransmitted, but electromagnetic radiation having a non-zero E field atthis location will be absorbed by the thin metal absorber layer. Asshown in FIG. 3B, strong absorbance of electromagnetic radiation in theorange to red light emitting range (i.e., 450 nm to 700 nm) shows thesuccessful application of this absorbing principle. As a result of usinga thin Cr absorbing layer at the requisite position, the reflectancespectra shown in FIG. 3C is achieved. The reflectance spectra in FIG. 3Cshows a strong, singular peak of electromagnetic radiation reflectanceat wavelengths from around 350 nm to around 500 nm, which is blue lightemitting electromagnetic radiation. This strong, singular reflectance ofblue emitting light is achieved, in part, by absorbing electromagneticradiation at wavelengths from 450 nm to 700 nm with a thin Cr absorberlayer.

Using metal layers, such as the aluminum core reflective layer andchromium absorber layer in a multilayer thin film structure providesoptical effects similar to a thirty one layer thin film structure usingonly dielectric layers. It was also found that by strategically usingmetal layers in the multilayer thin film structure, a seven layer thinfilm structure could be made to have a hue shift in the Lab color spacesimilar to that of a thin film having thirty one dielectric layers. Inparticular, seven layer thin film structures having a hue shift of lessthan 30°, in the Lab color space when viewed at angles from 0° to 45°can be achieved. Thus, using metal materials as layers in multilayerthin film structures significantly decreases the production time, cost,and efficiency by only requiring deposition of seven layers as opposedto thirty one layers.

Nanoencapsulation methods for forming multilayer thin film structuresaccording to embodiments will now be described with reference to FIG. 4.While the embodiments depicted in FIG. 4 are directed to forming a sevenlayer thin film structure, it should be understood thatnanoencapsulation methods disclosed and described herein can be used toform multilayer thin films having any number of desired layers.Nanoencapsulation methods for forming multilayer thin film structuresaccording to embodiments begin with a reflective core particle 110. Thisreflective core particle may be a discrete particle having any shape. Inembodiments, the reflective core particle 110 can have a thickness from10 nanometers (nm) to 5000 nm (i.e., 5 microns (μm)), such as from 50 nmto 1000 nm, from 100 nm to 600 nm, from 125 nm to 400 nm, from 150 nm to300 nm, or from 175 nm to 250 nm. In embodiments, the reflective coreparticle 110 can have a length from 5 μm to 100 μm, such as from 10 μmto 50 μm, or from 20 μm to 30 μm. In embodiments, the reflective coreparticle 110 can at least one of a “gray metallic” material, such as Al,Ag, Pt, Sn; at least one of a “colorful metallic” material, such as Au,Cu, brass, bronze, TiN, Cr, or a combination thereof.

The first nanoencapsulation step for forming multilayer thin filmstructures comprises forming a dielectric layer 120 that directlyencapsulates the reflective core particle 110. The dielectric layer 120may be deposited on the reflective core particle 110 by any suitablemethod, such as, for example, CVD, ALD, wet chemical processes, and PVD.The dielectric layer 120 can, according to embodiments, have a thicknessfrom 5 to 500 nm, such as from 50 nm to 500 nm, from 100 nm to 500 nm,from 150 nm to 500 nm, from 200 nm to 500 nm, from 250 nm to 500 nm,from 300 nm to 500 nm, from 350 nm to 500 nm, from 400 nm to 500 nm, orfrom 450 nm to 500 nm. In some embodiments, the dielectric layer 120 canhave a thickness from 5 nm to 450 nm, such as from 5 nm to 400 nm, from5 nm to 350 nm, from 5 nm to 300 nm, from 5 nm to 250 nm, from 5 nm to200 nm, from 5 nm to 150 nm, from 5 nm to 100 nm, or from 5 nm to 50 nm.In embodiments, the dielectric layer 120 can have a thickness from 50 nmto 450 nm, such as from 100 nm to 400 nm, from 150 nm to 350 nm, or from200 nm to 300 nm. In embodiments, the dielectric layer 120 can be madefrom at least one colorful dielectric material such as Fe₂O₃, TiN, or acombination thereof. In other embodiments, the dielectric layer 120 maybe a dielectric material selected from the group consisting of ZnS,ZrO₂, CeO₂ HfO₂, TiO₂, or combinations thereof. According to someembodiments, the dielectric layer 120 may be selected from ZnS, Fe₂O₃,TiO₂, or combinations thereof. In embodiments, the dielectric layer 120is comprised of one or more metal oxides. It should be understood thatcommercially available metal particles coated with a dielectric layermay be used in place of the first nanoencapsulation step.

The second nanoencapsulation step according to embodiments for forming amultilayer thin film structure comprises depositing an absorber layer130 that directly encapsulates the dielectric layer 120 (and in turnindirectly encapsulates the reflective core particle 110). The absorberlayer 130 may be deposited on the dielectric layer by any suitablemethod, such as ALD, PVD, CVD, or wet chemical processes. The absorberlayer 130 can, in embodiments, have a thickness from greater than 0 nmto 50 nm, such as from 1 nm to 40 nm, from 2 nm to 30 nm, from 3 nm to20 nm, from 4 nm to 20 nm, from 5 nm to 20 nm, from 10 nm to 20 nm, orfrom 15 nm to 20 nm. In embodiments, the absorber layer 130 can have athickness from 5 nm to 15 nm, such as from 5 nm to 10 nm, or from 10 nmto 15 nm. In embodiments, the absorber layer 130 can be made from atleast one material selected from W, Cr, Ge, Ni, stainless steel, Pd, Ti,Si, V, TiN, Co, Mo, Nb, ferric oxide, amorphous silicon, or combinationsthereof. In embodiments, the absorber layer 130 is comprised of one ormore metals.

The third nanoencapsulation step according to embodiments for forming amultilayer thin film structure comprises depositing an outer layer 140that directly encapsulates the absorber layer 130 (and in turnindirectly encapsulates the dielectric layer 120 and the reflective coreparticle 110). The outer layer 140 may be deposited by any suitablemethod, such as, for example, CVD, ALD, wet chemical processes, and PVD.The outer layer 140 can, in embodiments, have a thickness greater than0.1 quarter wave (QW) to less than or equal to 4.0 QW where the controlwavelength is determined by the target wavelength at the peakreflectance in the visible wavelength, such as between 0.5 QW and 4.0QW, between 1.0 QW and 4.0 QW, between 1.5 QW and 4.0 QW, between 2.0 QWand 4.0 QW, between 2.5 QW and 4.0 QW, between 3.0 QW and 4.0 QW, orbetween 3.5 QW and 4.0 QW. In embodiments, the outer layer 140 can havea thickness from greater than 0.1 QW to less than 3.5 QW, such as fromgreater than 0.1 QW to less than 3.0 QW, from greater than 0.1 QW toless than 2.5 QW, from greater than 0.1 QW to less than 2.0 QW, fromgreater than 0.1 QW to less than 1.5 QW, from greater than 0.1 QW toless than 1.0 QW, or from greater than 0.1 QW to less than 0.5 QW. Insome embodiments, the outer layer 140 can have a thickness from 0.5 QWto 3.5 QW, such as from 1.0 QW to 3.0 QW, or from 1.5 QW to 2.5 QW. Inembodiments, the target wavelength may be about 1050 nm. The outer layer140 can be made from a dielectric material with a refractive indexgreater than 1.6 such as ZnS, ZrO₂, CeO₂ HfO₂, TiO₂, or combinationsthereof. In some embodiments, the outer layer can be made from Fe₂O₃. Inembodiments, the outer layer is comprised of metal oxides.

In embodiments it may be beneficial if all of the layers of themultilayer thin film structure (which may comprise metal oxides andmetals) are deposited directly over the reflective core particle by thesame process. Accordingly, in some embodiments for forming multilayerthin film structures, the three nanoencapsulation steps described abovemay be conducted by a single process, such as where all threenanoencapsulation steps are conducted by ALD or PVD. However, in otherembodiments, the three nanoencapsulation steps described above may beconducted by different processes, such as where the firstnanoencapsulation step is conducted by, for example, CVD, the secondnanoencapsulation step is conducted by, for example, ALD, and the thirdnanoencapsulation step is conducted by PVD. It should be understood thatin embodiments where different deposition processes are conducted forthe nanoencapsulation steps, any combination of deposition processes maybe used in the nanoencapsulation steps for forming a multilayer thinfilm structure.

According to embodiments, a multilayer thin film structure may comprisean aluminum reflective core particle 110, a dielectric layer 120comprising TiO₂ (rutile phase or anatase phase) directly encapsulatingthe reflective core particle 110, a W or Cr absorber layer 130 directlyencapsulating the dielectric layer 120, and a TiO₂ (rutile phase andanatase phase) outer layer 140 directly encapsulating the absorber layer130. In embodiments, the absorber layer 130 may be W. By changing thelayer thickness and, thereby, absorber position, the multilayer thinfilm structure can reflect a variety of electromagnetic radiation withinthe visible spectrum.

According to some embodiments, a multilayer thin film structure maycomprise an aluminum reflective core particle 110, a dielectric layer120 comprising Fe₂O₃ (hematite) directly encapsulating the reflectivecore particle 110, a W or Cr absorber layer 130 directly encapsulatingthe dielectric layer 120, and a TiO₂ (rutile phase and anatase phase)outer layer 140 directly encapsulating the absorber layer 130. Inembodiments, the absorber layer 130 may be W. This structure isparticularly directed to reflecting electromagnetic radiation atwavelengths at or around 700 nm (near red light emitting electromagneticradiation). Compared to other colors (such as blue, green, or yellow)the available range of hue space is much narrower for red color. Becauseof this, the angular sensitivity requirement for red-colored multilayerthin film structures is much tighter and more challenging than for othercolors. Thus, multilayer thin film structures designed to reflectelectromagnetic radiation in the wavelength band that emits red requirenot only a “selective” absorber, such as Fe₂O₃ to reduce the angularsensitivity, but precise control of all the layers that are stacked intothe multilayer thin film structure is also maintained.

With reference again to FIG. 1B, the layers used to form a multilayerthin film structure 100 that provides ominidirectional structural colorfor black may, according to embodiments, comprise: a reflective coreparticle 110, such as Al; a dielectric layer 120 made from Fe₂O₃ thatdirectly encapsulates the reflective core particle 110; an absorberlayer 130 made from W that directly encapsulates the dielectric layer120 (and thereby encapsulates the reflective core particle 110); and anouter layer 140 made from Fe₂O₃ that directly encapsulates the absorberlayer 130 (and thereby encapsulates the dielectric layer 120 and thereflective core particle 110). However, deposition of the outer layer140, which is made from Fe₂O₃, by CVD or ALD generally comprises anoxidative agent, such as, for example ozone, that oxidizes underlyingabsorber layer 130. If a significant portion of the absorber layer 130is oxidized, such as by forming WO_(x), the absorber layer 130 may notfunction properly. Accordingly, in embodiments, steps are taken toprevent oxidation of the absorber layer 130.

An embodiment of a multilayer thin film structure that preventsoxidation of the absorber layer is provided with reference to FIG. 5.The multilayer thin film structure 500 according to embodiments shown inFIG. 5 may be considered as a nine layer thin film structure andcomprise: a reflective core particle 110; a dielectric layer 120 madefrom Fe₂O₃ that directly encapsulates the reflective core particle; anabsorber layer 130 made from W that directly encapsulates the dielectriclayer 120 (and thereby indirectly encapsulates the reflective coreparticle 110); a protective layer 135 made from Al₂O₃ or SiO₂ thatdirectly encapsulates the absorber layer 130 (and thereby indirectlyencapsulates the dielectric layer 120 and the reflective core particle110); and an outer layer 140 that directly encapsulates the protectivelayer 135 (and thereby indirectly encapsulates the absorber layer 130,the dielectric layer 120, and the reflective core particle 110).

In embodiments, the reflective core particle 110, the dielectric layer120, the absorber layer 130, and the outer layer 140 may have theproperties (e.g., thickness, length, etc.) of the correspondingcomponents disclosed above, and the dielectric layer 120, the absorberlayer 130, and the outer layer 140 may be formed by any of the methodsdisclosed above. In embodiments, protective layer 135 may be depositedon the absorber layer 130 by any suitable method, such as ALD, CVD, wetchemical processes, or PVD. The protective layer 135 can, inembodiments, have a thickness from greater than 0 nm to 50 nm, such asfrom 1 nm to 40 nm, from 2 nm to 30 nm, from 3 nm to 20 nm, from 4 nm to20 nm, from 5 nm to 20 nm, from 10 nm to 20 nm, or from 15 nm to 20 nm.In embodiments, the protective layer 135 can have a thickness from 5 nmto 15 nm, such as from 5 nm to 10 nm, or from 10 nm to 15 nm. Inembodiments, the protective layer 135 can be made from at least onematerial selected from Al₂O₃ or SiO₂. In embodiments, the protectivelayer 135 is comprised of Al₂O₃. A protective layer as described hereinwill, in embodiments, prevent the absorber layer 130 from oxidizing whenan outer layer 140 made from, for example, Fe₂O₃ is deposited on themultilayer thin film structure.

Embodiments of the multilayer thin film structures 100 and 500 describedabove have a hue shift of less than 30°, such as less than 25°, lessthan 20°, less than 15°, or less than 10° in the Lab color space whenviewed at angles from 0° to 45°.

With reference to FIG. 1B, in one or more embodiments, the multilayerthin film 100 comprises a reflective core particle 110 made from Almetallic material, a dielectric layer 120 made from Fe₂O₃ that directlyencapsulates the reflective core particle 110, an absorber layer 130made from W directly encapsulating the dielectric layer 120, and anouter layer 140 made from TiO₂ directly encapsulating the absorber layer130. This multilayer thin film 100 has a hue shift of less than 30°,such as less than 25°, less than 20°, less than 15°, or less than 10° inthe Lab color space when viewed at angles from 0° to 45°.

With reference to FIG. 1B, in one or more embodiments, the multilayerthin film 100 comprises a reflective core particle 110 made from Almetallic material, a dielectric absorber layer 120 made from TiO₂ thatdirectly encapsulates the reflective core particle 110, an absorberlayer 130 made from W directly encapsulating the dielectric layer 120,and an outer layer 140 made from TiO₂ directly encapsulating theabsorber layer 130. This multilayer thin film 100 has a hue shift ofless than 30°, such as less than 25°, less than 20°, less than 15°, orless than 10° in the Lab color space when viewed at angles from 0° to45°.

With reference to FIG. 1B, in one or more embodiments, the multilayerthin film 100 comprises a reflective core particle 110 made from Almetallic material, a dielectric absorber layer 120 made from Fe₂O₃ thatdirectly encapsulates the reflective core particle 110, an absorberlayer 130 made from W directly encapsulating the dielectric layer 120,and an outer layer 140 made from Fe₂O₃ directly encapsulating theabsorber layer 130. This multilayer thin film 100 has a hue shift ofless than 30°, such as less than 25°, less than 20°, less than 15°, orless than 10° in the Lab color space when viewed at angles from 0° to45°.

With reference to FIG. 1B, in one or more embodiments, the multilayerthin film 100 comprises a reflective core particle 110 made from Almetallic material, a dielectric absorber layer 120 made from TiO₂ thatdirectly encapsulates the reflective core particle 110, an absorberlayer 130 made from W directly encapsulating the dielectric layer 120,and an outer layer 140 made from TiO₂ directly encapsulating theabsorber layer 130. This multilayer thin film 100 has a hue shift ofless than 30°, such as less than 25°, less than 20°, less than 15°, orless than 10° in the Lab color space when viewed at angles from 0° to45°.

With reference to FIG. 1B, in one or more embodiments, the multilayerthin film 100 comprises a reflective core particle 110 made from Almetallic material, a dielectric absorber layer 120 made from ZnS thatdirectly encapsulates the reflective core particle 110, an absorberlayer 130 made from Cr directly encapsulating the dielectric layer 120,and an outer layer 140 made from ZnS directly encapsulating the absorberlayer 130. This multilayer thin film 100 has a hue shift of less than30°, such as less than 25°, less than 20°, less than 15°, or less than10° in the Lab color space when viewed at angles from 0° to 45°.

With reference to FIG. 1B, in one or more embodiments, the multilayerthin film 100 comprises a reflective core particle 110 made from Almetallic material, a dielectric absorber layer 120 made from Fe₂O₃ thatdirectly encapsulates the reflective core particle 110, an absorberlayer 130 made from Cr directly encapsulating the dielectric layer 120,and an outer layer 140 made from ZnS directly encapsulating the absorberlayer 130. This multilayer thin film 100 has a hue shift of less than30°, such as less than 25°, less than 20°, less than 15°, or less than10° in the Lab color space when viewed at angles from 0° to 45°.

With reference to FIG. 1B, in one or more embodiments, the multilayerthin film 100 comprises a reflective core particle 110 made from Almetallic material, a dielectric absorber layer 120 made from Fe₂O₃ thatdirectly encapsulates the reflective core particle 110, an absorberlayer 130 made from Cr directly encapsulating the dielectric layer 120,and an outer layer 140 made from TiO₂ directly encapsulating theabsorber layer 130. This multilayer thin film 100 has a hue shift ofless than 30°, such as less than 25°, less than 20°, less than 15°, orless than 10° in the Lab color space when viewed at angles from 0° to45°.

With reference to FIG. 5, in one or more embodiments, the multilayerthin film 500 comprises a reflective core particle 110 made from Almetallic material, a dielectric absorber layer 120 made from Fe₂O₃ thatdirectly encapsulates the reflective core particle 110, an absorberlayer 130 made from W directly encapsulating the dielectric layer 120, aprotective layer 135 made from Al₂O₃ directly encapsulating the absorberlayer 130, and an outer layer 140 made from Fe₂O₃ directly encapsulatingthe protective layer 135. This multilayer thin film 500 has a hue shiftof less than 30°, such as less than 25°, less than 20°, less than 15°,or less than 10° in the Lab color space when viewed at angles from 0° to45°.

With reference to FIG. 5, in one or more embodiments, the multilayerthin film 500 comprises a reflective core particle 110 made from Almetallic material, a dielectric absorber layer 120 made from Fe₂O₃ thatdirectly encapsulates the reflective core particle 110, an absorberlayer 130 made from W directly encapsulating the dielectric layer 120, aprotective layer 135 made from SiO₂ directly encapsulating the absorberlayer 130, and an outer layer 140 made from Fe₂O₃ directly encapsulatingthe protective layer 135. This multilayer thin film 500 has a hue shiftof less than 30°, such as less than 25°, less than 20°, less than 15°,or less than 10° in the Lab color space when viewed at angles from 0° to45°.

According to embodiments, multilayer thin film structures disclosed anddescribed herein may be used in paints, polymers, polymers or coatings.In embodiments, the multi-layer thin film structures described hereinmay be incorporated into a liquid carrier, such as an organic orinorganic binder, and utilized in a paint or similar coating systemwhich may be applied to an article of manufacture, thereby imparting theomnidirectional reflectivity properties of the multilayer thin filmstructure to the article. In some embodiments, multilayer thin filmstructure may be dispersed in a polymer matrix such that the multilayerthin film structures are randomly oriented in the matrix. Thereafter,the paint, coating, or polymer comprising the multilayer thin filmstructure may be deposited on an article of manufacture by spraying,electrostatic charging, powder coating, and the like. The depositedcoating thereby imparting the reflectance or shimmer of the metalliccomponent or the omnidirectional reflectivity properties of themultilayer thin film structure to the article to which it is applied.

According to embodiments, at least one of paint binders and fillers canbe used and mixed with the pigments to provide a paint that displays anomnidirectional structural color. In addition, other additives may beadded to the multilayer thin film to aid the compatibility of multilayerthin film in the paint system. Exemplary compatibility-enhancingadditives include silane surface treatments that coat the exterior ofthe multilayer thin film and improve the compatibility of multilayerthin film in the paint system.

It is noted that the terms “substantially” and “about” may be utilizedherein to represent the inherent degree of uncertainty that may beattributed to any quantitative comparison, value, measurement, or otherrepresentation. These terms are also utilized herein to represent thedegree by which a quantitative representation may vary from a statedreference without resulting in a change in the basic function of thesubject matter at issue.

EXAMPLES

Embodiments will be further clarified by the following examples.

Example 1

A nanoencapsulation method for forming a seven layer thin film structurethat provides omnidirectional structural color red pigments is provided.In this example, ALD was chosen for deposition of the absorber layer,which is formed from a metal, and the outer layer, which is formed froma metal oxide, due to the advantages of ALD, such as accurate thicknesscontrol, low temperature process, capability of continuous conformalcoating. The metal material chosen for the absorber layer is W(tungsten) due to availability of the ALD precursors, and the metaloxide material chosen for the outer layer is TiO₂ (titanium oxide).Commercially available Fe₂O₃ (iron oxide) coated aluminum (Al) flakepigments are used as substrates. The iron oxide is deposited on the Alreflective core particle using CVD and the results are summarized below

FIG. 6A shows original pigment flakes (Al reflective core layerencapsulated by an Fe₂O₃ dielectric layer) subject to different ALDcycles of W ranging from 0 to 40 cycles. As the number of ALD cycles ofW increases, the color of the resulting pigment changes from itsoriginal red to purple and eventually turns dark grey at 40 cycles. Thethickness of the deposited layer under different cycles is shown in FIG.6B and summarized in FIG. 6C. These figures clearly show that the Wmetal layer is continuously deposited over the Fe₂O₃ dielectric layer,and the thickness of the W metal layer is proportional to the cyclenumber, indicating that ALD can perform precise control of singleelement deposition, while a thin WO_(x) layer stays nearly at about 3 nmindependent of cycle numbers. Without being bound to any particulartheory, this is possibly due to passivation. Growth rate of the ALDdeposited W metal layer is about 13 nm/hr or 0.66 nm/cycle. FIG. 6Dshows the matched optical property (refractive index n and extinctioncoefficient k) of the deposited W metal layer with data reported over400 to 800 nm.

ALD was also used in this example to deposit a well-controlled TiO₂outer layer with high refractive index encapsulating the W metal layer(and thereby encapsulating the Fe₂O₃ dielectric layer and the Alreflective core particle). A crystalline phase of a TiO₂ formed layercan be controlled by ALD process parameters such as temperature. FIG. 7Ashows the temperature effects on the crystalline phase changing fromamorphous to anatase and later rutile, with increased temperature from180° C. to 400° C., while other processes such as wet-chemical methodusually require over 700° C. to achieve necessary crystalline phase.Anatase phase was chosen for this example due to the relatively mildprocess temperature required (250° C.). The lattice fringes of thedeposited layer in the middle HRTEM of FIG. 7A and the representativepeaks indicated by red arrows in the XPS spectra shown in FIG. 7Bclearly show the formation of the anatase phase. FIG. 7C shows thematched optical property (refractive index n and extinction coefficientk) of the deposited layer with reported data over 350 to 1000 nm. FIG.7D further shows the thickness is proportional to the cycle number withgrowth rate of about 0.052 nm/cycle, indicating ALD can perform precisecontrol of TiO₂ layer.

The above confirms precise control of nanometer scale layers of metalsand metal oxides can be deposited by ALD. Below we utilize ALD to createa multilayer thin film structure that provides red omnidirectionalstructural color and no compatibility issues have been identified amongdeposited layers or between deposited materials and substrates.

Preparation of a seven layer angle insensitive red reflective color thinfilm structure is shown in FIG. 8A, over the Al reflective coreparticle, a dielectric absorber layer like Fe₂O₃ is used to improve theangular insensitivity. Commercially available Fe₂O₃ coated Al particlesmay be used as core materials coated with Fe₂O₃. A W metal absorberlayer and TiO₂ outer layer are deposited in sequence over the Fe₂O₃layer to achieve the desired red color via the ALD process developedabove. To address the difference between theoretical optical propertiesand those in actual deposited layer over reflective core particles, amatrix to cover a broad range of W and TiO₂ layer thicknesses isprovided in FIG. 8B with different cycle numbers. FIG. 8C shows thereflectance spectra of the available samples (highlighted in red color)with different W and TiO₂ cycles (thus different layer thickness). FIG.8C clearly shows the color response towards different layer thickness ofTiO₂ and W. The increased TiO₂ layer thickness would result in a slowright-shift of red peak and the increase of unwanted blue peak.Meanwhile, an increase of W layer thickness can result in a right shiftof whole spectra and less reflectance. FIG. 8D shows the cross-sectionalimage of half stack of one representative flake.

The results in this example show the processes disclosed and describedherein are able to deposit both thin layers of metal and metal oxidesover reflective core particles (such as Al) with precise thicknesscontrol and desired optical properties. It opens up an opportunity toexplore new functions of structural color and to bring down the cost ofthe pigments.

Example 2

A nanoencapsulation method for forming a protective layer according toembodiments is provided. FIG. 9A shows the scheme of a seven layer thinfilm structure black omnidirectional structural color design thatconsists of layers of Fe₂O₃ and tungsten on Aluminum flakes. In thesynthesis scheme, commercially available Fe₂O₃ coated Al reflective coreparticles were obtained and ALD was used to deposit a precise layer of aW absorber directly on the Fe₂O₃ coated Al reflective core particle. Inone example ALD was also used to deposit an outer layer of Fe₂O₃ on theW absorber layer, and in another example CVD is used to deposit an outerlayer of Fe₂O₃ on the W absorber layer. However, during the Fe₂O₃deposition of the outer layer, either by ALD or CVD the thin W absorberlayer was prone to irreversible damage, e.g. subjected to oxidation neartungsten-iron oxide interface during ALD deposition of Fe₂O₃ in thepresence of oxidative ozone gas as shown in FIG. 9B, or react with acidduring wet process as shown in FIG. 9C.

In the gas phase deposition of Fe₂O₃, such as CVD and ALD when oxidativeagent ozone is normally used, an Al₂O₃ protective layer was applied,which is stable in an ozone environment and is easy to be deposited byALD. Two samples were prepared, one was tungsten coated particles andthe other Al₂O₃ (about 20 nm thick) encapsulated tungsten coatedparticles. Both samples were exposed to 700 cycles of ozone dosing,similar as the condition used for iron oxide deposition as shown in FIG.10A. Surface-sensitive technique X-ray photoelectron spectroscopy (XPS)was used to characterize the samples after test. W 4f XPS spectraclearly shows strong peaks of tungsten oxide in the unprotected sample,while there is no such formation and in the protected sample as shown inFIGS. 10A-10C.

These results show the addition of a thin protective layer is effectiveto prevent ultrathin tungsten layer from damage during iron oxidedeposition. It paves the way for iron oxide deposition to synthesizeLIDAR reflective black pigment.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the embodiments describedherein without departing from the spirit and scope of the claimedsubject matter. Thus it is intended that the specification cover themodifications and variations of the various embodiments described hereinprovided such modification and variations come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A multilayer thin film structure comprising: areflective core particle; a dielectric layer directly encapsulating thereflective core particle; an absorber layer directly encapsulating thedielectric layer; an outer layer encapsulating the absorber layer,wherein the multilayer thin film structure has a hue shift of less than30° in the Lab color space when viewed at angles from 0° to 45°.